The invention is directed to improvements in fuel injection systems of the distributor injection pump type for internal combustion engines.
The electrically controlled valves used in a known fuel injection apparatus for this type, which are usually electromagnetic valves, have substantially constant switching times that are determined by the valve construction. For accurate metering of the fuel injection quantity, the rpm and under some conditions the instant of injection as well must be taken into account when the opening and closing instants of the valves are set. This is done in view of the fact that as a rule the switching speed, that is, the time required for the opening or closing of the valves, is constant, so that during the phase of calculating the fuel injection quantity the two switching events affect the accuracy of the calculation, because the varying rpm. Attempts have therefore been made to use valves having the shortest possible switching time, so that the rpm error does not affect the calculation of the fuel injection quantity, or affects it only to a negligible extent.
The metering of the fuel injection quantity is also affected by variations from one valve to another in the valve type used. For example, the switching time of the valve can change over the service life of the valve, because of how the valve is constructed; over a long-term, drift can develop again negatively affecting the metering of the fuel injection quantity. Finally, such valves can also operate incorrectly, for example with sticking or seizing of the valve member, which depending on the situation may result in the destruction of the engine unless other safety precautions are taken. Such safety precautions are technologically possible but very expensive to use.
Injection nozzles used in connection with a fuel injection apparatus of the above generic type are also known in which the valve needle is electrically insulated with respect to its guide bore or the housing carrying it, is connected to a source of measuring voltage, and in its closing position has conductive contact via the valve seat with the electrically conductive housing of the injection valve, or with ground, which is connected to the other pole of the measuring voltage source.
With an injection nozzle equipped in this way, the injection onset is detected upon the opening of the injection nozzle, and via the injection nozzle a previously specified fuel injection quantity attains injection. The delivery of this fuel injection quantity is effected by the opening of the injection nozzle and keeps the nozzle needle in the open position for as long as the required injection pressure is maintained via the continuous delivery of fuel. The closure of the nozzle needle is effected by terminating the fuel delivery.